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Hurricanes: Science and Society

Since tropical cyclones usually spend the majority of their lives in the open ocean, far from the network of land-based weather observations, satellites (and aircraft, when available) often provide the best information about tropical cyclones. Prior to the existence of satellites, it was difficult to track tropical cyclones, and for those that never made landfall, sometimes their entire life cycles would be completely undetected. The first US weather satellite, TRIOS, was launched in 1960. Today, tropical cyclones can be observed with a variety of satellites with a wide array of instruments on board.

Satellite Orbits

A satellite is an object that moves around a larger object. The moon is a natural satellite of the Earth. Satellites that people launch into space are artificial satellites. The orbit is the path a satellite takes around the larger object. Satellites can be classified based on their orbit. There are two primary types of orbit used for weather and research satellites, geosynchronous orbits and low Earth orbits.

Geosynchronous orbits are orbits where the satellite moves around the Earth at the same rate as the Earth rotates, so that the satellite is located in the same place at the same time everyday. A special case of the geosynchronous orbit is the geostationary orbit, in which the satellite stays over the same exact spot on the Earth’s surface at all times. To achieve a geostationary orbit, a satellite must be located directly over the Earth’s equator at an altitude of approximately 35,784 kilometers (22,235 miles) above the Earth’s surface. Keeping a satellite in the same place over the Earth all the time is a great advantage for monitoring weather because the satellite can collect data from the same part of the Earth continuously.

Animation of a geosynchronous orbit. Notice that the satellite stays above the same point on Earth as the Earth rotates.

NOAA Satellite and Information Service.

Another advantage of the geosynchronous orbit is that the satellite is able to see about half of the Earth from its orbit. The satellite can collect information over a wide area.

One drawback to a geostationary orbit is the large distance from the Earth. At an altitude of 35,784 km (22,235 miles) above the Earth, some measurements are very hard to make.

Some weather satellites are placed into low Earth orbit. A low Earth orbit is much lower than a geostationary orbit, from 100 km to 1,500 km (or about 60 to 900 miles) above the Earth’s surface. One kind of low Earth orbit frequently used for weather and research satellites is a polar orbit. A polar orbit is an orbit with a path that takes the satellite near the Earth’s poles. A satellite in a polar orbit will orbit the Earth approximately once every 90 minutes. Each orbit will take it on a slightly different path across the Earth’s surface as the Earth rotates beneath it. A satellite in a polar orbit can view the whole Earth’s surface in one to two days.

Animation of a polar orbiting satellite. A polar orbit is a type of low Earth orbit.

NOAA Satellite and Information Service

Since it is closer to the Earth, a low Earth orbiting satellite can use instruments that a geostationary satellite cannot.

Satellites are often described by the instruments they carry. A single satellite may have several different instruments for measuring different properties of the Earth’s systems. The satellites that are used for weather research and forecasting often measure visible light, but they may also measure other parts of the electromagnetic spectrum, such as microwave radiation and infrared radiation. The instruments carried by weather satellites may be either passive or active instruments. Passive instruments measure only the radiation emitted or reflected from the Earth by natural processes. Active instruments emit radiation towards the Earth and then measure the reflection of that radiation from the Earth back to the satellite instrument.

The satellites primarily used for tropical cyclone observation, forecasting, and research are described in the following pages.

Geosynchronous Orbits - passive instruments

Low Earth Orbits - passive and active instruments